The present invention relates to ferritic stainless steel alloys. More particularly, the invention relates to an iron-chromium-aluminum alloy having additions of rare earth metals (hereafter referred to as “REM”).
The rare earth metals constitute a group of 15 chemically related elements in group IIIB of the Periodic Table (lanthanide series), namely, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. The primary commercial form of mixed rare earth metals is the so-called misch metal, prepared by the electrolysis of fused rare earth chloride mixtures.
In general, it is well-known that Fe—Cr—Al ferritic stainless steel is a material suitable for applications requiring high oxidation resistance, such as the catalyst substrate or carrier of an exhaust gas purifying device for automobiles.
U.S. Pat. No. 5,578,265 discloses a ferritic stainless steel alloy which can be used as a catalytic substrate. The alloy consists essentially of (by weight): 19-21% Cr, 4.5-6% Al, 0.01-0.03% Ce, with a total REM of 0.02-0.05%, >0.015% total Mg+Ca, and balance of Fe plus normally occurring impurities. The steel can be manufactured by producing a melt of the desired analysis, casting, hot rolling and cold rolling to thin sheets.
U.S. Pat. No. 4,414,023 discloses an iron-chromium-aluminum alloy with a REM addition, which alloy is resistant to thermal cyclic oxidation and hot workable. A preferred aluminum content between 3 to 8% is disclosed. Further, it is stated that there is a marked decline in the ability to texturize the aluminum oxide surface at aluminum contents above 8%, i.e., to form alumina whiskers.
Previous works have claimed that foil production by conventional rolling methods is impossible at Al contents higher than 5-8% Al. The further addition of Al is said to be very detrimental to the ductility and toughness of the material. In U.S. Pat. No. 5,045,404, it is disclosed that when the Al content is more than 6.5%, not only is the toughness of a hot rolled strip greatly lowered to thereby impair the processability, but also the thermal expansion coefficient becomes extremely high and leads to a serious amount of thermal fatigue due to the repeated heating and cooling effects when used as a catalyst carrier.
U.S. Pat. No. 5,228,932 describes a Fe—Cr—Al alloy having excellent oxidation resistance and high temperature brittleness resistance. The alloy consists of 10-28% Cr, 1-10% Al, additions of B, La and Zr and the balance Fe. At an Al content higher than 6%, it is disclosed that foil of this alloy cannot be produced by conventional methods. In this case, an alternative manufacturing method is employed. Al is added to the surface of the alloy by sputtering, cladding, etc. After this, the foil is homogenized by a heat treatment.
In view of the above prior art, there has been a prejudice against increasing the Al concentration to levels above 8% by weight, although this is desirable due to improved oxidation resistance when higher Al levels are present. The main reason for this reluctance to increasing the concentration of Al has resided in the assumption that an increase of the level of Al deteriorates the warm and cold workability such as warm and cold rolling to thin sheets.